d 44, I have just put keel cooling tubes along the bottom hull plating of my steel vessel. The reason I did so was to reinforce a large panel of bottom plating so that I could remove stringers which were acting as water traps and causing corrosion
I have not implemented keel cooling yet so I cant comment on how effective it may be. My engine is a Kubuto V2203, about 50 hp. I did not carry out any sort of calculations but wandered about in the boatyard looking at other keel coolers and discussed it with their owners. What I ended up with looked about right to me.
From a thermodynamic viewpoint there appear to be a number of things which require consideration:
The reason you put the swizzle stick down the centre of a heat exchanger
tube is to create turbulence in the fluid flowing through the tube which assists in presenting new, cooler fluid to the heat exchange face in the tube. Consequently it appears that a wide thin fluid conduit would provide better heat exchange than a round tube. If my rusty old maths is right your round tube will give you a circumference of about 121mm however if you were to split a piece of radius-ed corner 100 x 50 RHS you would exceed this and probably have a more turbulent coolant flow as well as having the coolant in contact with the considerable heat sink of the keel itself.
NahivanniV's comments re cleaner appearance and snagging and damage are valid and of considerable import
from a potential damage viewpoint.
I used 76 x 3mm wall thickness tube but split the circumference into thirds and threw one third away using the other two as the cooling conduits, looks rather neat.
I don't know how to post images
on here but there are some in the Raymond Rampling site on Facebook.